The present invention pertains to the field of arrangements for maintaining the operational status of an electrical system during short-term power dropouts.
Systems which derive their operating voltages from the AC power line are susceptible to temporary interruptions of their operation due to short-term power line dropouts which occur, for example, when lightning strikes near a power line.
Power line dropouts tend to annoy a user even if shot term, since information related to particular operating conditions of the system may be lost during the power line dropout necessitating that the user reset the system to the prior operating conditions after power returns. This is particularly the case with systems employing digital electrical control arrangements rather than mechanical ones since the latter arrangements have inherent memory capabilities.
Not only are dropouts annoying, but when line power returns, the system may be set to undesirable operating conditions. For example, in a television receiver in which information related to whether the receiver is off or on is stored in a memory cell, when power returns after a dropout, the memory cell may be set to a state corresponding to the on condition of the receiver even though the receiver was intentionally turned off before the dropout occurred. While arrangements are known for resetting digital control arrangements to a predetermined state when power returns, the predetermined state may not necessarily correspond to the condition of the system before the power line dropout occurred. As a result, the user may be annoyed by having to return the system to its previous condition.
A variety of standby power supply arrangements are known for maintaining the operation of a system or information related to operating conditions of a system during power line dropouts. Typically, these employ a power storage element such as a battery or a capacitor. Batteries are undesirable since they require replacement by the user or recharging circuitry. While relatively small value capacitors may be employed as power storage elements in systems which have digital control apparatus requiring relatively small amounts of operating power, relatively large value capacitors are needed for storage elements in systems which have digital control apparatus requiring relatively large amounts of operating power. Since the storage capacity of a capacitor is related to its size, the use of a capacitor as a power storage element can be undesirable. Moreover, both batteries and relatively large value capacitors tend to be expensive.